This valve (usually a red octagonal button) is mounted in the cab of the vehicle, easily accessible to the driver. The driver opens the valve by pushing or pulling the button, depending on the type used.

Opening the valve permits main reservoir pressure to flow through the valve. This pressure is piped to the tractor protection valve and the supply line glad hand. The valve is spring loaded, and will be held in the open position when sufficient pressure is reached. If the pressure drops to a range of between 45 – 20 P.S.I., the valve will shut automatically by spring pressure, opening the exhaust port. The driver can close the valve manually to uncover the exhaust port.